This invention relates to the structural configuration of the cellular portions of a honeycomb structure, and more particularly to improved cellular structures having flexible expansion joints which resist thermal shock breakage.
Cellular or honeycomb structures made of a ceramic material have application as substrates in catalytic converters for the emissions from internal combustion engines. Due to the extreme temperature variations which such honeycomb structures are subjected to, it is imperative that the structures be provided with the highest thermal shock resistance possible. Naturally, it is desirable to utilize a material having a low coefficient of expansion, and to strengthen the web portions of the structure as much as possible to maintain their integrity. However, in view of the fact that the thickness of such web portions varies between about 0.002 inch and 0.050 inch so as to provide open frontal areas of about 75% or greater, and further in view of the fact that the materials utilized are necessarily of a porous nature so as to increase surface area and facilitate the adhesion of a catalyst thereon, the amount of strengthening and the number of acceptable compositions are severely limited. However, by incorporating proper web geometry (i.e. the configuration of the partition members forming the cells) it is possible to increase the thermal shock resistance by selecting a cell configuration which will provide improved thermal shock resistance through the utilization of movable joints or intersections of the cell walls, which joints absorb or compensate for compressive and tensile stresses generated in the body during thermal expansion and contraction thereof.
Honeycomb ceramic structures, which may be utilized as substrates in catalytic converters, are readily formed by extrusion, such as by utilizing the method and apparatus disclosed in U.S. Pat. No. 3,790,654. The composition of the extruded material may vary according to the desired properties to be obtained, however low expansion ceramic compositions are disclosed in U.S. Pat. No. 3,885,977, which have particular application in the formation of ceramic honeycomb structures for use as catalytic support matrices in emission control devices. Such matrices are formed with parallel passages or cells extending longitudinally therethrough with a minimum amount of cross sectional wall thickness, so as to provide open frontal areas of about 75% or greater. In order to provide uniform flow through such passageways and avoid "short circuiting", it is necessary that the cell structure forming such passages be uniform across the areal extent of the honeycomb matrix. That is, the cell structure forming the longitudinal passages should be uniform in both size and shape when viewed perpendicularly to the longitudinal axis of the honeycomb article, so that the flow front will experience substantially uniform impedance across the face of the honeycomb matrix.
Catalytic honeycomb support structures utilized in emissions control are generally housed within a casing or container such as shown in U.S. Pat. Nos. 3,801,289 and 3,841,839 forming a part of the exhaust system of an internal combustion engine. In view of the temperature of the exhaust gases which pass through the honeycomb structure and the temperatures generated by the catalytic action, the ceramic honeycomb structure is subjected to wide temperature variations causing expansion and contraction thereof within the outer housing or casing. As a result, the structure is subjected to substantial radial compressive forces during expansion caused by heat-up, and to a lesser degree tensile forces during cool-down. Therefore, although ceramic materials having relatively low coefficients of thermal expansion are utilized in extruding the honeycomb structures for use in catalytic converters, such structures are relatively brittle, and in order to inhibit the cracking or breaking-up of the ceramic core members due to thermal shock, it is necessary to provide the interconnecting webs or walls forming the cells with movable expansion joint means which can tolerate large strains without breaking.
It thus has been an object of the present invention to provide novel cell geometries for honeycomb structures which provide improved thermal shock resistance.